View的getDrawingCache为空,解决办法
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这两天帮同事解决一个问题;
View.getDrawingCache获得数据始终为null,但是在某些设备上并不为null,纠结够 久啊,网上说了一些原因:
1) (mViewFlags & WILL_NOT_CACHE_DRAWING) == WILL_NOT_CACHE_DRAWING 这个值为true
2) (mViewFlags & DRAWING_CACHE_ENABLED) == DRAWING_CACHE_ENABLED 为false,buildDrawingCache没执行
3) buildDrawingCache执行失败
这些在源码中都可以看到,在获得缓存数据的时候,跟背景色(drawingCacheBackgroundColor),透明度isOpaque,use32BitCache这些有关系,看是细看这些东西都是表面的,是系统在buildDrawingCache的时候,根据View或都系统设置而来的;有些属性是不能更改的;这样一来当一个固定大小的View在不同的设备上生成的图片就可能有所不同,我同事这边存在的问题就是,设置View的固定大小为1360*768,而我的设备分辨率为1024*600,而源码里可以看到这样代码:
if (width <= 0 || height <= 0 || // Projected bitmap size in bytes (width * height * (opaque && !use32BitCache ? 2 : 4) > ViewConfiguration.get(mContext).getScaledMaximumDrawingCacheSize())) { destroyDrawingCache(); mCachingFailed = true; return; }
当我们在buildDrawingCache的时候,系统给了我们默认最大的DrawingCacheSize为屏幕宽*高*4;而我的View的CacheSize大小超过了某些设备默认值,就会导致获得为空;开始想着用反射的方法去改变这些属性,或者设置背景颜色来改变图片质量,这样一来CacheSize大小 就可能会变小,但是这样始终不能达到效果;
最终解决方案:
查看系统buildDrawingCache方法可以看到:
public void buildDrawingCache(boolean autoScale) { if ((mPrivateFlags & DRAWING_CACHE_VALID) == 0 || (autoScale ? mDrawingCache == null : mUnscaledDrawingCache == null)) { mCachingFailed = false; if (ViewDebug.TRACE_HIERARCHY) { ViewDebug.trace(this, ViewDebug.HierarchyTraceType.BUILD_CACHE); } int width = mRight - mLeft; int height = mBottom - mTop; final AttachInfo attachInfo = mAttachInfo; final boolean scalingRequired = attachInfo != null && attachInfo.mScalingRequired; if (autoScale && scalingRequired) { width = (int) ((width * attachInfo.mApplicationScale) + 0.5f); height = (int) ((height * attachInfo.mApplicationScale) + 0.5f); } final int drawingCacheBackgroundColor = mDrawingCacheBackgroundColor; final boolean opaque = drawingCacheBackgroundColor != 0 || isOpaque(); final boolean use32BitCache = attachInfo != null && attachInfo.mUse32BitDrawingCache; if (width <= 0 || height <= 0 || // Projected bitmap size in bytes (width * height * (opaque && !use32BitCache ? 2 : 4) > ViewConfiguration.get(mContext).getScaledMaximumDrawingCacheSize())) { destroyDrawingCache(); mCachingFailed = true; return; } boolean clear = true; Bitmap bitmap = autoScale ? mDrawingCache : mUnscaledDrawingCache; if (bitmap == null || bitmap.getWidth() != width || bitmap.getHeight() != height) { Bitmap.Config quality; if (!opaque) { // Never pick ARGB_4444 because it looks awful // Keep the DRAWING_CACHE_QUALITY_LOW flag just in case switch (mViewFlags & DRAWING_CACHE_QUALITY_MASK) { case DRAWING_CACHE_QUALITY_AUTO: quality = Bitmap.Config.ARGB_8888; break; case DRAWING_CACHE_QUALITY_LOW: quality = Bitmap.Config.ARGB_8888; break; case DRAWING_CACHE_QUALITY_HIGH: quality = Bitmap.Config.ARGB_8888; break; default: quality = Bitmap.Config.ARGB_8888; break; } } else { // Optimization for translucent windows // If the window is translucent, use a 32 bits bitmap to benefit from memcpy() quality = use32BitCache ? Bitmap.Config.ARGB_8888 : Bitmap.Config.RGB_565; } // Try to cleanup memory if (bitmap != null) bitmap.recycle(); try { bitmap = Bitmap.createBitmap(width, height, quality); bitmap.setDensity(getResources().getDisplayMetrics().densityDpi); if (autoScale) { mDrawingCache = bitmap; } else { mUnscaledDrawingCache = bitmap; } if (opaque && use32BitCache) bitmap.setHasAlpha(false); } catch (OutOfMemoryError e) { // If there is not enough memory to create the bitmap cache, just // ignore the issue as bitmap caches are not required to draw the // view hierarchy if (autoScale) { mDrawingCache = null; } else { mUnscaledDrawingCache = null; } mCachingFailed = true; return; } clear = drawingCacheBackgroundColor != 0; } Canvas canvas; if (attachInfo != null) { canvas = attachInfo.mCanvas; if (canvas == null) { canvas = new Canvas(); } canvas.setBitmap(bitmap); // Temporarily clobber the cached Canvas in case one of our children // is also using a drawing cache. Without this, the children would // steal the canvas by attaching their own bitmap to it and bad, bad // thing would happen (invisible views, corrupted drawings, etc.) attachInfo.mCanvas = null; } else { // This case should hopefully never or seldom happen canvas = new Canvas(bitmap); } if (clear) { bitmap.eraseColor(drawingCacheBackgroundColor); } computeScroll(); final int restoreCount = canvas.save(); if (autoScale && scalingRequired) { final float scale = attachInfo.mApplicationScale; canvas.scale(scale, scale); } canvas.translate(-mScrollX, -mScrollY); mPrivateFlags |= DRAWN; if (mAttachInfo == null || !mAttachInfo.mHardwareAccelerated || mLayerType != LAYER_TYPE_NONE) { mPrivateFlags |= DRAWING_CACHE_VALID; } // Fast path for layouts with no backgrounds if ((mPrivateFlags & SKIP_DRAW) == SKIP_DRAW) { if (ViewDebug.TRACE_HIERARCHY) { ViewDebug.trace(this, ViewDebug.HierarchyTraceType.DRAW); } mPrivateFlags &= ~DIRTY_MASK; dispatchDraw(canvas); } else { draw(canvas); } canvas.restoreToCount(restoreCount); canvas.setBitmap(null); if (attachInfo != null) { // Restore the cached Canvas for our siblings attachInfo.mCanvas = canvas; } } } /** * Create a snapshot of the view into a bitmap. We should probably make * some form of this public, but should think about the API. */ Bitmap createSnapshot(Bitmap.Config quality, int backgroundColor, boolean skipChildren) { int width = mRight - mLeft; int height = mBottom - mTop; final AttachInfo attachInfo = mAttachInfo; final float scale = attachInfo != null ? attachInfo.mApplicationScale : 1.0f; width = (int) ((width * scale) + 0.5f); height = (int) ((height * scale) + 0.5f); Bitmap bitmap = Bitmap.createBitmap(width > 0 ? width : 1, height > 0 ? height : 1, quality); if (bitmap == null) { throw new OutOfMemoryError(); } Resources resources = getResources(); if (resources != null) { bitmap.setDensity(resources.getDisplayMetrics().densityDpi); } Canvas canvas; if (attachInfo != null) { canvas = attachInfo.mCanvas; if (canvas == null) { canvas = new Canvas(); } canvas.setBitmap(bitmap); // Temporarily clobber the cached Canvas in case one of our children // is also using a drawing cache. Without this, the children would // steal the canvas by attaching their own bitmap to it and bad, bad // things would happen (invisible views, corrupted drawings, etc.) attachInfo.mCanvas = null; } else { // This case should hopefully never or seldom happen canvas = new Canvas(bitmap); } if ((backgroundColor & 0xff000000) != 0) { bitmap.eraseColor(backgroundColor); } computeScroll(); final int restoreCount = canvas.save(); canvas.scale(scale, scale); canvas.translate(-mScrollX, -mScrollY); // Temporarily remove the dirty mask int flags = mPrivateFlags; mPrivateFlags &= ~DIRTY_MASK; // Fast path for layouts with no backgrounds if ((mPrivateFlags & SKIP_DRAW) == SKIP_DRAW) { dispatchDraw(canvas); } else { draw(canvas); } mPrivateFlags = flags; canvas.restoreToCount(restoreCount); canvas.setBitmap(null); if (attachInfo != null) { // Restore the cached Canvas for our siblings attachInfo.mCanvas = canvas; } return bitmap; }
生成DrawingCache的过程貌似就是利用获得View的Canvas然后画到bitmap上,直接返回对应 的bitmap,这样一来,就是我们用getDrawingCache获得的bitmap;跟我们直接将View画到bitmap貌似区别 不是很大,受启发;如下:
自己生成Bitmap;
public static Bitmap loadBitmapFromView(View v, boolean isParemt) {if (v == null) {return null;}Bitmap screenshot;screenshot = Bitmap.createBitmap(v.getWidth(), v.getHeight(), HDConstantSet.BITMAP_QUALITY);Canvas c = new Canvas(screenshot);v.draw(c);return screenshot;}
这样也就将View生成了我们需要的bitmap了,但是有些情况下:比如ViewPager在用getDrawingCache和我自己生成的Bitmap时候,会有区别,ViewPager第一屏是正常的,滑动到第二屏幕的时候,我手动生成的Bitmap不见了,而系统getDrawingCache方法生成 的Bitmap是可见的,郁闷,,,详细看了一下系统buildDrawingCache访求,发现在Canvas绘制Bitmap之后,多了一个步骤:
computeScroll(); final int restoreCount = canvas.save(); canvas.scale(scale, scale); canvas.translate(-mScrollX, -mScrollY);
很明显,系统Canvas,对默认位置进行了移动,即启发:我们在用哥滑动View获得它的Bitmap时候,获得的是整个View的区域(包括隐藏的),如果想得到当前区域,需要重新定位到当前可显示的区域;自己的代码修改:
public static Bitmap loadBitmapFromView(View v, boolean isParemt) {if (v == null) {return null;}Bitmap screenshot;screenshot = Bitmap.createBitmap(v.getWidth(), v.getHeight(), HDConstantSet.BITMAP_QUALITY);Canvas c = new Canvas(screenshot);c.translate(-v.getScrollX(), -v.getScrollY());v.draw(c);return screenshot;}
完美解决用自己生成 的Bitmap替换系统的getDrawingCache()方法;
当然系统getDrawingCache()考虑的因素很多,这一些我们也可以自己直接定义,比如透明磁,大小 ,图片质量等;最重要就是
c.translate(-v.getScrollX(), -v.getScrollY());这行一代码需要理解 ;
上面个人见解,如有理解不对的地方,大神留言指导啊........
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